Electric power inverter with adaptive third harmonic auxiliary impulse commutation

ABSTRACT

In a 3-phase third harmonic auxiliary impulse commutated electric power inverter, during a commutation interval the firing signal for the oncoming main valve of the inverter is delayed for a programmed interval of fixed duration after the commutation capacitor voltage changes polarity, whereby the peak voltage on the capacitor can automatically vary with the magnitude of load current.

CROSS REFERENCE TO RELATED APPLICATIONS

Certain features of the illustrated embodiment of this invention are theclaimed subject matter of copending patent applications Ser. No. 679,980and Ser. No. 679,979 which we filed concurrently herewith and assignedto General Electric Company.

BACKGROUND OF THE INVENTION

This invention relates to electric power inverters for converting directcurrent (d-c) to polyphase alternating current (a-c), and moreparticularly it relates to improvements in the old and well knowncurrent-fed "third harmonic" auxiliary impulse commutated inverters. Theprinciples of commutation and a typical practical application of suchinverters were described in a technical paper entitled: "Analysis of aNovel Forced-Commutation Starting Scheme for a Load-CommutatedSynchronous Motor Drive," which paper was presented by R. L. Steigerwaldand T. A. Lipo at the IEEE/IAS annual meeting held in Los Angeles,Calif. on Oct. 2-4, 1977. The Steigerwald and Lipo paper was reprintedin IEEE TRANS. Vol. IA-15, No. 1, January/Febuary 1979, pgs. 14-24, andit is expressly incorporated herein by reference.

In essence, a third harmonic auxiliary impulse commutated invertercomprises six main unidirectional conduction controllable electricvalves, such as thyristors, that are interconnected in pairs of seriesaiding, alternately conducting valves to form a conventional 3-phase,double-way, 6-pulse bridge between a pair of d-c terminals and a set ofthree a-c terminals. The d-c terminals of the bridge are adapted to beconnected to a suitable source of relatively smooth direct current. Alarge, multicell, heavy duty electric storage battery. is a suitablecurrent source, as is the combination of an electric power rectifier towhich an alternating voltage source is connected and a current smoothingreactor or choke in the d-c link between the d-c terminals of therectifier and inverter, respectively. The a-c terminals of the aforesaidbridge are respectively connected to the different phases of a 3-phaseelectric load circuit which typically comprises star-connected 3-phasestator windings of a dynamoelectric machine such as a large synchronousmotor.

To supply the load circuit with 3-phase alternating current, the sixmain valves of the inverter are cyclically turned on (i.e., renderedconductive) in a predetermined sequence in response to a family of"firing" signals (gate pulses) that are periodically generated in aprescribed pattern and at desired moments of time by associated controlmeans. To periodically turn off the main valves by forced commutation,the inverter is provided with an auxiliary circuit comprising aprecharged commutation capacitor and at least seventh and eighthalternately conducting unidirectional controllable electric valves thatare arranged to connect the capacitor between the neutral or commonpoint of the 3-phase a-c load circuit and either one of the d-cterminals of the bridge.

During each full cycle of steady state operation of a third harmonicinverter, each of the valves in the auxiliary commutation circuit isbriefly turned on three separate times. More particularly, the 7th valveis fired at intervals of approximately 120 electrical degrees, and the8th valve is fired at similar intervals that are staggered with respectto the intervals of the 7th valve, whereby one or the other auxiliaryvalve is fired every 60 electrical degrees. When an auxiliary. valve isturned on, the commutation capacitor is effectively placed in parallelwith one phase of the load circuit and a first one of the two mainvalves which are then conducting load current. Initially, the capacitorvoltage magnitude is higher than the amplitude of the line-to-neutralvoltage that is developed across the inductive load, and its polarity issuch that the capacitor starts discharging. Consequently current isforced to transfer (commutate) from the first main valve (i.e., theoffgoing or relieved valve) to a parallel path including the turned-onauxiliary valve and capacitor. The rate of change of current duringcommutation will be limited by the load inductance.

After current in the offgoing main valve decreases to zero, themagnitude of capacitor voltage is still sufficient to keep that valvereverse biased for longer than its "turn-off time." As soon as thecommutation capacitor is fully discharged, load current beginsrecharging it with opposite polarity. Once the commutation capacitor isrecharged to a voltage magnitude exceeding that of the line-to-neutralload voltage, the next (oncoming) main valve in the bridge is forwardbiased and can be turned on, whereupon load current commutates from theturned-on auxiliary valve and commutation capacitor to the oncoming mainvalve. This causes the auxiliary valve to turn off and completes thecommutation process. The capacitor is left with voltage of properpolarity and sufficient peak magnitude for successful commutation of thesecond one of the first-mentioned two conducting main valves when theopposite auxiliary valve is turned on approximately 60 degrees later. Itwill be apparent that there are six intervals of commutation per cycle,the direction of current in the commutation capacitor during eachinterval is reversed compared to the preceding interval, and thereforethe fundamental frequency of the alternating capacitor current equalsthe third harmonic component of load frequency.

As is pointed out in the referenced Steigerwald and Lipo paper, onepractical application of a current-fed third harmonic auxiliarycommutated inverter is in an adjustable speed a-c drive system where the3-phase star-connected stator windings of a synchronous machine aresupplied with variable frequency a-c power by the inverter which needsto be forced commutated in order to start the machine. In such anapplication, for reasons explained in that paper, a technique of"delayed gating" is used to ensure that at the end of each commutationinterval the commutation capacitor has recharged to a sufficiently highlevel of voltage to guarantee successful commutation during thesucceeding interval. According to this technique, the sequential firingsignals for the main valves are each delayed, after the oncoming valveis forward biased, while the capacitor continues accumulating andstoring electrostatic charge until its voltage attains a threshold levelrequired for extinguishing current in the next offgoing valve.Steigerwald and Lipo suggest that the threshold level can beproportional to the magnitude of source current so that the peakmagnitude of capacitor voltage (and hence the commutating ability of theinverter) will desirably track the demands of the load. In other words,the magnitude of capacitor voltage is high when necessary to commutatehigh current and is relatively low when only light load needs to becommutated. This "adaptive" commutation capacitor voltage techniqueadvantageously reduces commutation time and power losses during thecommutation intervals when the magnitude of load current is relativelylow.

In his prior art U.S. Pat. No. 4,244,017, Steigerwald discloses andclaims a modified third harmonic auxiliary impulse commutated inverterhaving parallel commutation circuits which allow three different valuesof commutating capacitance to be actively selected as a function of themagnitude of load current. Assuming that the inverter is supplying asynchronous machine load, current tends to decrease as frequency (i.e.,rotor speed) and hence machine back emf increase. By. switching to acommutation capacitor of smaller size when the current magnitude fallsbelow a preset level, the commutation time is desirably shortened atlight loads. As a result, the maximum permissible fundamental frequencyis increased, and the operating range of the inverter is extended.

A current-fed third harmonic inverter is well suited for supplyingvariable frequency alternating current to the 3-phase stator windings ofa rotatable synchronous machine that is used to start or "crank" a primemover such as a large internal-combustion engine. In such a system, therotor of the machine is coupled to a mechanical load comprising thecrankshaft of the engine. Initially the output torque of the rotor (andhence the magnitude of current in the stator windings) needs to berelatively high in order to start turning the crankshaft. As the rotoraccelerates from rest, less torque (and current) will be required, whilethe fundamental frequency of load current increases with speed(revolutions per minute). In its cranking mode of operation, theinverter supplies the machine with current of properly varying magnitudeand frequency until the engine crankshaft is rotating at a rate thatequals or exceeds the minimum speed at which normal running conditionsof the engine can be sustained. It should be apparent that theabove-mentioned adaptive commutation technique, wherein decreasingcurrent is accompanied by lower commutation voltage and hence shortercommutation intervals, will desirably raise the upper limit of thepermissible range of inverter operating frequency.

SUMMARY OF THE INVENTION

A general objective of the present invention is to provide a thirdharmonic auxiliary. impulse commutated electric power invertercharacterized by an improved adaptive commutation feature that is simpleyet effective.

Another objective is the provision, for such an inverter, of an adaptivecommutation feature that does not require means for sensing or measuringcurrent magnitude.

In carrying out the invention in one form, a source of relatively smoothdirect current is connected to a 3-phase inductive load circuit by meansof an electric power inverter comprising at least three pairs ofalternately conducting main controllable electric valves arranged in a3-phase, double-way, 6-pulse bridge configuration. For commutating themain valves, a precharged commutation capacitor is connected between theload circuit and the juncture of first and second auxiliary controllableelectric valves that are interconnected in series aiding fashion acrossthe d-c source. Bistable voltage sensing means is coupled to thecapacitor; it has one state whenever the electrical potential on oneside of the capacitor is measurably positive with respect to the otherside, and otherwise it has a different state. A zero-crossing detectoris coupled to the 3-phase load circuit for detecting all zero crossingsof the fundamental phase-to-phase alternating voltages that aredeveloped at line terminals of the respective phases of the load.

The above-summarized inverter also comprises control means coupled toboth the voltage sensing means and to the zero-crossing detector forcyclically producing a family of periodic firing signals that cause thevalves to turn on selectively. In a third harmonic commutation mode ofoperation, the aforesaid family includes a first series of firingsignals respectively produced in response to consecutive zero crossingsof the phase-to-phase voltages for alternately turning on the first andsecond auxiliary valves, whereupon load current can immediately transferfrom an offgoing main valve to a parallel path including the turned-onauxiliary valve and the commutation capacitor which is first dischargedand then recharged with reverse polarity by this pulse of current. Thefamily also includes a second series of firing signals respectivelyproduced in delayed response to successive state changes of the voltagesensing means for turning on the main valves in a predeterminedsequence, whereupon load current can then transfer to the oncoming mainvalve from the turned-on auxiliary valve. In accordance with the presentinvention, the control means includes time delay means effective atleast after a predetermined initial period of time, which starts whenthe third harmonic commutation mode of operation commences, for delayingthe production of each firing signal in the second series until aprogrammed interval of time has elapsed following each state change ofthe voltage sensing means. During the programmed delay interval, loadcurrent will continue to recharge the commutation capacitor at a ratethat is a function of its magnitude. Preferably, each delay interval hasa fixed, relatively short duration. As a result, during each commutationinterval the capacitor is automatically charged to a peak voltage levelthat varies with current magnitude. Thus, as load current decreases thecommutation intervals are desirably reduced and the upper frequencylimit of the inverter is correspondingly increased.

The invention will be better understood and its various objects andadvantages will be more fully appreciated from the following descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system comprising a rotatableelectrical machine of the synchronous type having a rotor which ismechanically coupled to a variable speed prime mover and having 3-phase,star-connected stator windings which are connected to an electricstorage battery via a plurality of controllable electric valves that inturn are interconnected and arranged to form a variable frequency thirdharmonic auxiliary impulse commutated inverter;

FIG. 1A is a simplified block diagram of the controller (shown as asingle block in FIG. 1) which cyclically produces a family of periodicfiring signals for respectively turning on the various valves of theinverter;

FIG. 2 a time chart showing, for one full cycle of operation in a thirdharmonic commutation mode, the six possible states of 3-phasefundamental stator voltages and the family of twelve firing signalsproduced by the controller;

FIG. 3 is a larger scale time chart showing variations in voltage andcurrent of the commutation capacitor during transitions from odd to evenstates and from even to odd states, and also showing the programmeddelay intervals of the present invention; and

FIGS. 4-11 are flow charts that explain the operation of the preferredembodiment of the FIG. 1A controller to produce the firing signals shownin FIGS. 2 and 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The electric power system shown in FIG. 1 comprises a third harmonicauxiliary impulse commutated inverter having a pair of d-c terminals 1Opand 1On connected to a source of relatively smooth direct current and aset of three a-c terminals 11, 12, and 13 connected, respectively, toline terminals of three star-connected armature windings on the statorof a rotatable, variable speed, 3-phase a-c synchronous machine 14 whichhas a rotor 15 that is mechanically coupled to a prime mover 16. In theillustrated embodiment of the invention, the current source for theinverter comprises the combination of a source of voltage, such as aheavy duty electric storage battery 17, in series with impedance meanswhich has appreciable electrical inductance, preferably provided by thed-c field winding 18 on the rotor 15 of the machine 14. By way ofexample, the battery 17 is a lead-acid or nickel-cadmium type having 32cells and rated 68 volts, and the average magnitude of voltage at itsterminals normally does not exceed 76 volts. Its internal resistance istypically in the range of 16 to 37 milliohms. The battery is intended tosupply electric energy for starting the prime mover, and the systemshown in FIG. 1 can successfully perform this function with the batteryvoltage as low as 61 volts.

The prime mover 16 can be a conventional thermal or internal-combustionengine, and in one particular application of the invention it is ahigh-horsepower, 16-cylinder diesel engine that is used to provide themotive power on a large self-propelled diesel-electric traction vehiclesuch as a locomotive. The synchronous machine 14 has dual modes ofoperation: as a generator for supplying alternating current to anelectric load circuit that is connected to its stator windings, and asan a-c motor for cranking or starting the engine 16. In its generatingmode, the rotor 15 of the machine is driven by the crankshaft of theengine 16, and the field winding 18 is energized by a suitableexcitation source 20 (e.g., the rectified output of auxiliary windingson the stator of the machine 14) to which it is connected by means of asuitable contactor K which is closed by a conventional actuatingmechanism 21 on command. The machine 14 now generates alternatingvoltages at the line terminals of its 3-phase stator windings. The rmsmagnitude of the fundamental sinusoidal components of these voltagesdepends on the angular velocity (rpm) of the rotor and on the amount offield excitation. The generated voltages are applied to a-c inputterminals of at least one 3-phase, double-way rectifier bridge 22, andthe rectified electric power at the output terminals of each such bridgeis supplied via a d-c bus to one or more d-c traction motors (notshown). As is shown in FIG. 1, the bridge 22 comprises simplesolid-state diodes, but alternatively it could be a controlled rectifierif desired. As is suggested by the broken lines 23a and 23b, anadditional traction motor (not shown) could be connected between the d-cterminals 1Op and 10n if desired.

In the motoring mode of operation, which is assumed throughout theremainder of this description, the rotor 15 of the synchronous machine14 drives the crankshaft of the engine 16. Now electric energy issupplied from the battery 17 to the windings on both the rotor and thestator of the machine, and the rotor 15 exerts torque to turn thecrankshaft and thereby crank the engine. As the rotor accelerates fromrest, both the frequency and the rms magnitude of the fundamentalalternating voltage waveforms developed at the line terminals of thestator windings (i.e., the back emf) correspondingly increase, whileload current (i.e., current in the field and armature windings)decreases in magnitude. Once the rotor is rotating faster than apredetermined rate, which typically is 240 rpm, the engine is presumedto be started and the motoring mode (i.e., engine cranking mode) ofoperation is discontinued. Assuming that the machine 14 has ten poles,240 rpm corresponds to a fundamental frequency of 20 Hertz. Thus thefundamental frequency of alternating current supplied to the statorwindings of the machine 14 needs to increase from 0 to approximately 20Hz in order for the illustrated system to perform its engine startingfunction.

The previously mentioned third harmonic auxiliary impulse commutatedinverter is operative to convert direct current from the battery 17 intovariable frequency alternating currents in the three different phases A,B, and C of the 3-phase armature windings on the stator of the machine14. The inverter has at least three pairs of alternately conducting maincontrollable electric valves interconnected and arranged in a 3-phase,double-way bridge configuration between the set of three a-c terminals11, 12, and 13 and the pair of d-c terminals 1Op and 1On. Moreparticularly: a first pair of valves T1 and T4 are connected in seriesaiding fashion from terminal 1Op to terminal 1On, and their juncture,comprising terminal 11, is connected to phase A of the stator windings;a second pair of valves T3 and T6 are connected in series aiding fashionfrom 1Op to 1On, and their juncture, comprising terminal 12, isconnected to phase B of the stator windings; and a third pair of valvesT5 and T2 are connected in series aiding fashion from 1Op to 1On, andtheir juncture, comprising terminal 13, is connected to phase C. Eachvalve preferably comprises at least one solid state unidirectionalcontrolled rectifier popularly known as a thyristor. It has a turned on(conducting) state and a turned off (non-conducting) state. In practicethe valves are respectively shunted by conventional snubber circuits(not shown). The illustrated means for connecting the d-c terminals 1Opand 1On of the inverter to the battery 17 will next be described in moredetail.

The first d-c terminal 1Op is connected to the relatively positiveterminal of the battery 17 via a single pole contactor K1p, and thesecond d-c terminal 1On is connected to the other terminal of thebattery by means of a conductor 25, one pole K3a of a double-polecontactor K3, the field winding 18, the other pole K3b of the samecontactor, and a conductor 26. The field winding 18 typically has aresistance in the range of 0.12 to 0.28 ohm and an unsaturatedinductance of more than 0.3 henry. A single-pole contactor K1n, aconductor 27, and resistance means comprising two resistors 28 and 29are connected in parallel circuit relationship with the field winding 18in the load current path between conductors 25 and 26. The resistors 28and 29 are in series, and both have very low ohmic values; for example,the resistance of resistor 28 is approximately 14 milliohms and theresistance of resistor 29 is approximately 23 milliohms. The secondresistor 29 is shunted by another single-pole contactor K2 which, whenclosed, reduces the ohmic value of the resistance means to that of thefirst resistor 28 alone.

An inductor 30 of approximately one millihenry inductance is connectedin series with resistors 28 and 29 between the second resistor 29 andthe conductor 26 to smooth the current flowing in this branch of theload current path. The inductor 30 is shunted by a conventionalovervoltage protective device 31 the resistance of which is normallyvery high but automatically decreases to a negligible amount insubstantially instantaneous response to the magnitude of voltage acrossthe inductor rising to a predetermined breakover level (e.g., 750volts). A similar protective device 32 with bidirectional response isconnected across the field winding 18. A resistor 36 of significantohmic value (e.g., 100 ohms) is also connected across the winding 18 toenable thyristor "latching" current to bypass the field and the inductor30 when battery current starts flowing to precharge the inverter'scommutation capacitor as will later be explained.

As is shown in FIG. 1, a battery charger 33, in series with a blockingdiode 34 and a circuit breaker 35, is connected across the combinationof battery 17 and inductor 30. With the engine 16 running understeady-state conditions, the battery charger holds the battery voltageat approximately 74 volts. It can be energized from any suitable source,such as auxiliary windings (not shown) on the stator of the synchronousmachine 14.

With the field winding 18 in the load current path during enginecranking, the synchronous machine 14 will operate with a characteristicsimilar to that of a series d-c motor, namely, high current and hencedesirably high starting torque at low speeds. The resistance means 28,29 in parallel with the field reduces the ohmic value of resistance thatthe field winding alone would otherwise introduce in this path, therebyinitially allowing a higher magnitude of armature current and later, asspeed increases, providing automatic field weakening which permits themachine to run at a higher speed. Initially load current is limited bythe internal resistance of the battery 17 as well as other resistance inits path, and as speed increases it is limited by the back emf of thearmature (i.e., stator) windings. Thus load current and torque tend todecrease with increasing speed. A short time after cranking commences,the contactor K2 is closed to further reduce the amount of resistance inparallel with the field, thereby permitting more load current to flowand hence more torque to be developed at higher speeds compared to thequantities that would be achieved if the parallel resistance were not soreduced.

When the cranking mode of operation commences, the contactor K is open,and all of the contactors in the load current path between the battery17 and the d-c terminals 1Op and 1On are closed except K2. In a mannerthat will soon be explained, contactor K2 is commanded to close upon theexpiration of a predetermined length of time after cranking commences.Thereafter, in response to the speed of the engine attaining a threshold(e.g., 240 rpm) that marks the conclusion of cranking and therefore thesuccessful starting of the engine 16, all of the previously closedcontactors are opened. Upon opening contactor K3 the field winding 18 isdisconnected from the load current path between the conductors 25 and26, and the contactor K is then closed by its actuating mechanism 21 inorder to reconnect the field to the normal excitation source 20.

Each of the four contactors K1p, K1n, K2, and K3 has an associatedactuating mechanism that determines its closed or open status. All foursuch mechanisms are represented in FIG. 1 by a single block 38 labeled"Contactor Drivers," and they respectively respond to opening/closingsignals received over lines 40, 41, 42, and 43 from another block 44labeled "Controller." The controller 44 knows the actual status of eachcontactor by virtue of feedback signals that it receives fromconventional position indicators (not shown) that are associated withthe separable contact members of the respective contactors, asrepresented symbolically by broken lines in FIG. 1.

In order to turn on each of the controllable valves T1 through T6 in theinverter, an appropriate signal is applied to the associated gate whilethe main electrodes of that valve are forward biased (i.e., anodepotential is positive with respect to cathode). Such a signal issometimes called a trigger or gating signal, and it is herein referredto generically as a "firing signal." In a manner soon to be described,the controller 44 cyclically produces a series of periodic firingsignals for turning on the respective main valves T1-T6 in numericalorder. (It is assumed that the alternating voltages developed at theline terminals of the 3-phase stator windings of the machine 14 have theconventional A-B-C phase rotation.) In order to quench or turn off eachvalve when desired, the inverter has a forced commutation circuitincluding at least first and second auxiliary controllable electricvalves Tp and Tn interconnected in series aiding fashion between the d-cterminals 10p and 1On and connected via a commutation capacitor 45 tothe stator windings of the machine 14. The capacitor 45 is shunted by ableeder resistor 46 which effectively keeps the capacitor initially in asubstantially discharged state prior to closing the contactors K1p andK1n and starting up the illustrated system. Preferably, the commutationcapacitor is connected between the juncture M of the auxiliary valvesand the neutral S of the three star-connected stator windings.

In the manner previously explained under the heading "Background of theInvention," the main valves T1-T6 in turn are forced to turn off by thecommutation action that is initiated each time one or the other of theauxiliary valves Tp and Tn is turned on. The controller 44 is arrangedcyclically to produce a series of periodic firing signals foralternately turning on the two auxiliary valves in synchronism with thevariable frequency fundamental component of the sinusoidalphase-to-phase alternating voltages that are developed at the lineterminals of the respective phases A, B, and C of the stator windings asthe field winding 18 rotates inside the stator of the machine 14. Notethat the peak magnitude of reverse voltage imposed on the auxiliaryvalves can be reduced, if desired, by respectively inserting simplediodes in series therewith.

To produce the valve firing signals at proper times, the controller 44needs to receive from the power system information or data indicatingwhen the fundamental waveforms of line-to-neutral magnetic flux in thethree phases A, B and C of the machine 14 cross zero and changepolarity, and indicating the status of the electrostatic charge orvoltage on the commutation capacitor 45. Such data are supplied by meansof a voltage processor 38 which, as can be seen in FIG. 1, has aplurality of input wires respectively connected to the line terminals ofthe stator windings and to opposite sides of the capacitor 45. Insidethe processor 48 there is bistable first means for sensing theelectrical potential difference across the commutation capacitor.Whenever the potential at the juncture M is measurably positive withrespect to the neutral S, the first means is in one state and provides adiscrete signal (VE1) that is high or "1," but when this potential ismeasurably negative with respect to neutral the first means is in adifferent state in which the output signal VE1 is low or "0." Voltagesensors suitable for this purpose are well known and readily availableto a person skilled in the art. The signal VE1 is supplied over anoutput bus 50 to the controller 44. An additional bistable voltagesensing means is provided in the voltage processor 48 for detectingwhether or not the capacitor voltage has a magnitude exceeding apredetermined level, either positive or negative. In one practicalapplication of the illustrated system, the predetermined level is 400volts. The additional sensor produces a discrete signal (VE2) on theoutput bus 50. As the commutation capacitor charges or recharges to avoltage magnitude in excess of the predetermined level, the signal VE2changes from a "0" to a "1" state.

The voltage processor 48 also includes suitable means for integratingthe respective line-to-neutral voltages of the stator windings and forindicating whether the polarity of the integral is positive or negative.The latter means provides three discrete output signals XA, XB and XCwhich are respectively supplied over lines 51, 52 and 53 to thecontroller 44. The output signal XA is high or "1" only during the halfcycles that the time integral of the voltage between the line terminalof phase A and the neutral S is relatively positive. It will be apparentthat up and down changes of XA coincide with successive zero crossingsof both the magnitude of line-to-neutral flux in phase A and themagnitude of the fundamental phase-to-phase alternating voltagedeveloped at the stator line terminals of phases C and B (i.e., the lineterminals to which the a-c terminals 13 and 12 of the inverter arerespectively connected). Similarly, the output signal XB is "1" onlyduring the half cycles that the integral of the phase B-to-neutralvoltage is relatively positive, whereby up and down changes of XBcoincide with successive zero crossings of both the magnitude ofline-to-neutral flux in phase B and the magnitude of the fundamentalphase-A-to-phase-C alternating voltage developed at the stator lineterminals to which the a-c terminals 11 and 13 are connected. In asimilar manner, the output signal XC is "1" only during the positivehalf cycles of the integral of the phase C-to-neutral voltage, wherebythe up and down changes of XC coincide with successive zero crossings ofboth the magnitude of line-to-neutral flux in phase C and the magnitudeof the fundamental phase-B-to-phase-A alternating voltage developed atthe stator line terminals to which a-c terminals 12 and 11 arerespectively connected. By logically processing the resulting outputsignals XA, XB, and XC, the six different combinations of relativepolarities of the three phase-to-phase voltages are indicated duringeach cycle of operation. Each time the magnitude of any of thesevoltages crosses zero, a different one of the output signals changeseither from 0 to 1 or from 1 to 0.

The controller 44 also communicates with master controls 54 via inputand output busses 55 and 56. A starting switch 57 is associated with themaster controls 54. The starting switch 57 can be either a pushbuttontype or a turn-and-hold type.

The presently preferred embodiment of the controller 44 is shown in moredetail in FIG. 1A. Its main component is a microcomputer 60. Personsskilled in the art will understand that the microcomputer 60 is actuallya coordinated system of commercially available components and associatedelectrical circuits and elements that can be programmed to perform avariety of desired functions. It typically comprises a centralprocessing unit (CPU) which executes an operating program permanentlystored in a read-only memory (ROM) which also stores tables and datautilized in the program. Contained within the CPU are conventionalcounters, registers, accumulators, flag flip flops, etc. along with aprecision oscillator which provides a high-frequency clock signal. Themicrocomputer also includes a random access memory (RAM) into which datamay be temporarily stored and from which data may be read at variousaddress locations determined by the program stored in the ROM. The CPU,ROM, and RAM are interconnected by appropriate address, data, andcontrol busses. In one practical embodiment of the invention, an Intel8031 microprocessor is used.

The other blocks shown in FIG. 1A represent conventional peripheral andinterface components that interconnect the microcomputer 60 and theexternal circuits of FIG. 1. More particularly, block 61 is aninput/output circuit (I/O) for connecting the output bus and lines 50-53of the voltage processor 48 to the microcomputer 60, and block 62 isanother I/0 for connecting the microcomputer 60 to the contactor drivers38. Block 63 is suitable means for decoding the position indicators thatare respectively associated with the contactors K1p, K1n, K2, and K3.Block 64 is a gate pulse generator (GPG) and buffer that produces, oncommand of the microcomputer 60, properly shaped and isolated firingsignals that turn on the respective valves T1-T6, Tp, and Tn.

The operation of the controller 44 during engine cranking can best beunderstood with the aid of FIGS. 2 and 3. In FIG. 2, the sinusoidalwaveforms of the fundamental components of the three line-to-neutralvoltages V_(AS), V_(BS), and V_(CS) of the 3-phase stator windings ofthe machine 14 are depicted by solid-line traces for a full cycle ofsteady-state operation, and the integral of one such waveform (i.e.,phase A) is shown by the broken-line trace. This integral is known to bein phase with the flux of phase A. Assuming a symmetrical 3-phasemachine and balanced loading, the zero crossings of the integral ofV_(AS) are seen to coincide with the moments of equality between V_(BS)and V_(CS), that is, with the zero crossings of the instantaneousmagnitude of the fundamental phase-to-phase alternating voltage betweenthe line terminals of phases B and C. Consequently the discrete signalXA on output line 51 of the zero crossing detecting means in the voltageprocessor 48 is "1" throughout each half cycle of relatively positivepolarity of the phase C-to-phase B voltage and is "0" throughout eachrelatively negative half cycle thereof, the signal XB on the output line52 is "1" throughout each relatively positive half cycle of the phaseA-to-phase C voltage but is otherwise "0," and the output signal XC is"1" only during each relatively positive half cycle of the phaseB-to-phase A voltage. The six different states of these three signalsare marked off and numbered consecutively in FIG. 2. For example, state1 exists so long as both XA and XB but not XC are "1," whereas state 2exists while XB alone is "1." A state change is experienced each timeany one of the signals XA, XB, or XC changes up or down, and each statecoincides with a different 60segment of a full cycle (360 electricaldegrees) of the fundamental component of alternating voltage.

In a manner that will soon be described, in its third harmoniccommutation mode of operation the controller 44 automatically respondsto consecutive state changes of the signals XA, XB, and XC by producinga series of firing signals (which are represented by the pointers 68 and69 in FIG. 2) for alternately turning on the two auxiliary valves Tp andTn. More particularly, the controller is effective to produce a firingsignal 69 for turning on the auxiliary valve Tn in immediate response toeach change from an odd numbered state to the succeeding even numberedstate (i.e., from state 1 to state 2, from state 3 to state 4, and fromstate 5 to state 6), and it is effective to produce a firing signal 68for turning on the auxiliary valve Tp in immediate response to eachchange from an even numbered state to the succeeding odd numbered state(i.e., from state 6 to state 1, from state 2 to state 3, and from state4 to state 5). By thus synchronizing the firing signals 68 and 69 withthe state changes (which are determined by the angular location of therotor 15 in the machine 14), the angle between the field mmf and thestator mmf of the machine is controlled.

As was previously explained, turning on an auxiliary valve causes loadcurrent immediately to transfer from an offgoing main valve to aparallel path including the turned-on auxiliary valve and the prechargedcommutation capacitor 45 which is first discharged and then rechargedwith reverse polarity by such current. As is indicated in FIG. 2, thepolarity of the capacitor voltage will change from positive (i.e., thepotential at the juncture M is positive with respect to the neutral S)to negative as a result of the auxiliary valve Tn being turned on by oneof the firing signals 69, and it will change from negative to positivewhen the auxiliary valve Tp is turned on by one of the firing signals68.

Following the production of each of the firing signals 68 and 69, thecontroller 44 selectively produces the next one of a series of sixfiring signals (represented in FIG. 2 by the pointers 71-76) which areapplied to the gates of the main valves T1-T6, respectively. Thecontroller selects the proper firing signal to turn on whichever one ofthe main valves is associated with the oncoming or relieving phase ofthe stator voltages, whereupon load current can then transfer to theoncoming valve from the turned-on auxiliary valve. More particularly, asindicated in FIG. 2, the controller selects the firing signal 71 forturning on the main valve T1 if the preceding state change was fromstate 6 to state 1, it selects the firing signal 72 for turning on themain valve T2 if the preceding state change was from state 1 to state 2,it selects the firing signal 73 for turning on the main valve T3 if thepreceding state change was from 2 to 3, it selects the firing signal 74for turning on the main valve T4 if the preceding state change was from3 to 4, it selects the firing signal 75 for turning on the main valve T5if the preceding state change was from 4 to 5, and it selects the firingsignal 76 for turning on the main valve T6 if the preceding state changewas from 5 to 6. Whichever one of the firing signals 71-76 is selected,it is not actually produced until after the first-mentioned bistablecapacitor voltage sensing means in the voltage processor 48 changesstate, as indicated by an up or down change of the discrete signal VE1on the output bus 50 of the processor 48. This is best seen in FIG. 3which will now be described.

FIG. 3 shows the instantaneous magnitudes of capacitor voltage (v_(MS))and current (i_(CAP)) during two consecutive commutation intervals. Thefirst of these two intervals is initiated at time t1 when the means fordetecting the zero crossings of phase-to-phase voltages changes from anodd state to an even state and the controller responds by producing afiring signal 69 to turn on the auxiliary valve Tn, and the secondcommutation interval is initiated at time t6 when a firing signal 68 forthe auxiliary. valve Tp is next produced in response to the samedetector changing from even to odd states. Once the first commutationinterval is initiated, current begins to increase in the auxiliary valveTn and in the commutation capacitor 45, while current in the offgoingmain valve decreases to zero at time t2 which occurs when all of theload current has transferred to the parallel commutation circuit. Theresulting pulse of current in the commutation capacitor first dischargesit and then recharges it with reverse polarity. At times t3 thecapacitor is fully discharged, whereupon the discrete signal VE1 changesfrom its initial "1" state to a different state. The time from t2 to t3is the circuit turn Off lime during which the offgoing main valverecovers its ability to withstand reapplied forward voltage. The nextone (7X) of the series of firing signals 71-76 for the main valves T1-T6is produced in response to the 1-to-0 change of VE1.

In accordance with the present invention, the controller 44 includestime delay means effective at least after a predetermined initial periodof time for delaying the production of the next firing signal 7X until aprogrammed interval of time has elapsed following the 1-to-0 change ofVE1. This interval is designated by the delta t- symbol in FIG. 3, andit elapses at time t4. Now the oncoming main valve TX is turned on, andload current begins transferring to it from the parallel commutationcircuit while continuing to recharge the capacitor 45. At time t5 all ofthe load current has transferred to the oncoming main valve, theauxiliary valve Tn turns off, and the first commutation interval isfinished. The peak magnitude of capacitor voltage (i.e., its magnitudeat time t5) is a function of the magnitude of load current thatrecharges the capacitor during the delta t- delay interval. As loadcurrent decreases, the peak magnitude of capacitor voltage willdecrease, and consequently the length of the commutation interval isdesirably reduced. From t5 to t6 the commutation capacitor will retain avoltage of sufficient magnitude and proper polarity (negative) to ensuresuccessful commutation when the next zero crossing of phase-to-phasevoltages initiates the second commutation interval.

As is apparent in FIG. 3, the second commutation interval is essentiallya dual of the first. In this case the programmed delay interval (i.e.,the time from the 0-to-1 change of the discrete signal VE1 to theproduction of the firing signal 7X for turning on the next oncoming mainvalve) is designated delta t+, and its duration can either be the sameas or differ from the duration of delta t-. In practice, the programmerwill select delay intervals that are compatible with the parameters ofthe power system and the ratings of its components. In the preferredembodiment, after a predetermined initial period of time from the startof engine cranking, alternate delay intervals (delta t-) are programmedto have a shorter duration than intermediate delay intervals (delta t+).

FIGS. 4 through 11 display flow charts of the presently preferredprograms that are executed by the microcomputer 60 in the controller 44in order to produce firing signals that enable the inverter to operatein a third harmonic commutation mode for purposes of cranking the engine16. The Main Routine is shown in FIG. 4. It begins at the entry pointlabeled "Start." When commanded to start, the first step 80 of the MainRoutine is to initialize the various inputs to the microcomputer 60, toreset its timers to 0, to decrement its counters to 0, and to set thestack pointers, registers, latches, outputs, and variable values of themicrocomputer to their respective quiescent states or normal levels atthe start of the first pass through the Main Routine. Upon completingthis initializing step, the program determines, at a decision point 81,whether or not the start switch 57 is "on." Assuming the start switch isturned on or closed (which will happen at a time when the rotor of thesynchronous machine 14 is at rest and the commutation capacitor 45 isdischarged), the control proceeds to a step 82 in which a first timer isstarted. By way of example, this timer will run for approximately 90seconds after being started. Once timer #1 has been started, a Set-UpRoutine 83 is executed, and this is followed by the execution of aNormal Cranking Routine 84.

The Set-up Routine 83 is shown in FIG. 5. Its purpose is to control thecontactors and valves of the power system (FIG. 1) so as to: (1)precharge the commutation capacitor 45, (2) initiate a pulse ofexcitation current from the battery 17 through the field winding of themachine 14 and find the initial angular position of the rotor 15, and(3) ensure that the capacitor voltage has the right polarity forsuccessful third harmonic commutation once the Normal Cranking Routineis initiated. The Set-up Routine is entered at a point labeled "Set-up"and then proceeds to a step 85 which causes the controller 44 to issuesignals, via lines 40-43, that command the contactor actuatingmechanisms (38) to close the four contactors K1p, K1n, K2, and K3. Thisstep is followed by an inquiry, at point 86, as to the open or closedstatus of the contactors. Once all four are actually closed, a"Contactor Error" flag is set in an "off" state, and the control istransferred to a Capacitor Ring-up Subroutine 87 which will soon bedescribed. If all four contactors do not close in response to theclosing commands of step 85, they are commanded to open (step 88), theContactor Error flag is set in its "on" state, and the Set-up Routine isaborted at the stop point 89.

The presently preferred embodiment of the Capacitor Ring-up Subroutine87 is shown in FIG. 6. While this subroutine is being executed, thecontroller 44 will produce firing signals that cause the FIG. 1 systemto operate in a capacitor "ring-up" mode that precharges the commutationcapacitor. It is entered at a point labeled "Ring-up" and then proceedsto a step 91 which sets a counter in the microcomputer 60 at apredetermined maximum number of ring cycles (e.g., 20 cycles). Step 91is followed by a step 92 in which a second timer is started. By way ofexample, this timer will run for an interval of approximately 10milliseconds after being started. From step 92 the program proceeds toan inquiry point 93 where the state of the first bistable capacitorvoltage sensor in the voltage processor 48 is tested. If the commutationcapacitor has a measurably positive voltage (i.e., the juncture M has apositive potential with respect to the neutral S and the amount ofpotential difference exceeds a predetermined threshold such as 5 volts),the first voltage sensor is in one state (which is indicated by VE1being high) and the inquiry yields an affirmative answer. On the otherhand, if the capacitor voltage were measurably negative (i.e., thepotential at juncture M is more than 5 volts negative with respect tothe neutral S), the voltage sensor is in a different state (as indicatedby VE1 being low) and the answer to the inquiry is "no".

In response to an affirmative answer at the inquiry point 93, the nextstep 94 in the program is to instruct the controller's gate pulsegenerator 64 to generate a first pair of concurrent firing signals forthe auxiliary valve Tn and for a preselected complementary one of themain valves (e.g., T1). In response to a negative answer at 93, theCapacitor Ring-up Subroutine alternatively proceeds to a step 95 inwhich the gate pulse generator is instructed to generate a second pairof concurrent firing signals for the auxiliary valve Tp and for anotherpreselected complementary one of the main valves (e.g., T2). Inpractice, each firing signal that is generated in step 94 or 95 canactually comprise a burst of several high-frequency, short-durationdiscrete d-c signals having sufficient magnitude to turn on theassociated valve.

Upon turning on either the complementary pair of valves Tn and T1 or thecomplementary pair Tp and T2, battery current will begin flowing througha path which in FIG. 1 is seen to comprise: (1) the field winding 18 inparallel with both the resistor 36 and the series combination ofresistor 28, inductor 30 and the closed contactor K2, (2) one phase ofthe armature windings of the machine 14, and (3) the commutationcapacitor 45. Preferably this path has a sufficiently high Q so thatcurrent quickly oscillates from zero to a peak magnitude and back tozero, and in the process the capacitor is incrementally charged withreverse polarity. The conducting pair of valves will automatically turnoff by self commutation when current oscillates to zero at theconclusion of each cycle of this ringing action. The resulting pulse ofcurrent typically has a duration of less than two milliseconds.

In the Capacitor Ring-up Subroutine, the status of the second timer istested immediately after either step 94 or step 95. This testing step isindicated in FIG. 6 by the inquiry point 96. Assuming that timer #2 isstill running, the next step 97 in the program is to check the state ofthe second bistable capacitor voltage sensor in the voltage processor48. So long as the magnitude of capacitor voltage does not exceed apredetermined maximum (i.e., the level at which the output signal VE2 ofthe second voltage sensor changes from "0" to "1"), step 97 yields anegative answer, and the control returns to the preceding step 96.Whenever timer #2 stops running (i.e., its time delay interval is over),the control proceeds from step 96 to a step 98 in which the count storedin the cycle counter (see step 91) is reduced by one, and then to a step99 that determines whether or not the count has reached 0. If not, thecontrol returns to step 92, and the steps 92 through 99 are recycled. Inthis manner the controller repeatedly produces the aforesaid second pairof firing signals (for turning on Tp and T2) if it is determined in step93 that the capacitor voltage is not positive, and the aforesaid firstpair of firing signals (for turning on Tn and T1) if the voltage ispositive. The start of each such repeated cycle of operation is delayedby an interval of time determined by timer #2. This interval issufficiently long to enable the pulse of battery current to firstdischarge the commutation capacitor and then incrementally recharge itwith reverse polarity until the current oscillates to zero. At the endof each consecutive cycle, the electrostatic charge that is stored inthe capacitor (and hence the capacitor voltage) will be progressivelyincreased in magnitude due to the ringing nature of the charging currentpath (the inductance of which is provided by the field and armaturewindings of the machine 14), and it will have alternately positive andnegative polarity. This action continues for a sufficient number ofcycles to enable the magnitude of capacitor voltage to attain theaforesaid maximum at which the inquiry step 97 yields an affirmativeanswer, whereupon the second timer is stopped (step 101) and, afterwaiting a very short, fixed period of time (step 102), the controlproceeds to execute one final ring-up cycle of operation.

As is shown in FIG. 6, the final cycle of the Capacitor Ring-upSubroutine is carried out by steps 103, 104, and 105 which areduplicates of the previously described steps 93, 94, and 95,respectively. Following the generation of the last pair of concurrentfiring signals (either for the complementary valves Tn and T1 or for thecomplementary valves Tp and T2), timer #2 is cleared (step 106), andthen the control returns to the Set-up Routine (FIG. 5). The totalnumber of ring-up cycles that are carried out by the Capacitor Ring-upSubroutine is sufficient for the commutation capacitor to charge to avoltage magnitude many times (i.e., more than approximately five times)higher than the average magnitude of voltage at the terminals of thebattery 17. In one practical embodiment, the fully charged voltagerating of the battery 17 is 74 volts, and in four or five cycles thecommutation capacitor can be charged to a voltage magnitude exceeding400 volts. If for any reason the capacitor were not charged to thedesired level within the maximum number of ring cycles that was set atstep 91, the count in the cycle counter would reach 0 (step 99) beforethe second capacitor voltage sensor detects maximum voltage (step 97),and in this abnormal event all of the contactors are commanded to open(step 107) and a "No Ring-up" signal is issued (step 108).

After execution of the Capacitor Ring-up Subroutine 87, and with thecommutation capacitor now precharged, the Set-up Routine 83 continues asshown in FIG. 5. The next step 110 is to start a third timer. By way ofexample, this timer will run for an interval of approximately 250milliseconds after being started. From step 110 the control proceeds toa step 111 which causes the controller 44 to command the contactoractuating mechanisms to open both of the contactors K1n and K2. This isfollowed by testing, at point 112, the status of the third timer and bytesting, at point 113, the open or closed status of the contactors K1nand K2. As soon as both of these contactors actually open, but no laterthan the time at which timer #3 stops running, the control proceeds to astep 114 in which the gate pulse generator is instructed to generateconcurrent firing signals for turning on a preselected pair of valvesthat will provide a path for excitation current from the battery 17through the field winding 18 of the synchronous machine 14. Anyappropriate pair of valves can be turned on for this purpose. Theauxiliary valves Tp and Tn were selected in the illustrated embodiment.Once these valves are turned on, field current starts increasing orramping up from zero.

After generating the firing signals for the auxiliary valves Tp and Tn(step 114), the Set-up Routine proceeds to an inquiry point 115 wherethe status of timer #3 is tested again. If the time delay interval ofthis timer is over, the control proceeds from step 115 to a step 116 inwhich the contactor K1n is commanded to reclose. When K1n recloses, theseries resistors 28 and 29 are reconnected in parallel with the fieldwinding 18 (FIG. 1), thereby permitting a more rapid increase of currentin the auxiliary valves. Step 116 is followed by a step 117 whichintroduces an additional delay (e.g., a fixed period of approximately300 milliseconds) to allow excitation current in the field winding 18 tocontinue increasing. Upon the expiration of this additional delay (atwhich time current in Tp and Tn may have attained a magnitude as high as1,300 amperes), the control is transferred to a subroutine 118 whichwill soon be explained. Executing the subroutine 118 completes theSet-up Routine (FIG. 5), and the control will then return to the MainRoutine (FIG. 4).

The presently preferred embodiment of the subroutine 118 is shown inFIG. 7. While this subroutine is being executed, the controller 44 findsthe initial or at-rest position of the rotor 15, and it produces firingsignals that will turn on the proper main valve(s) for commutating(turning off) the auxiliary valves Tp and Tn and for obtaining the rightpolarity of vdltage on the commutation capacitor. Note that at the timethis subroutine is entered, excitation current is rising in the fieldwinding 18 on the rotor 15 of the machine 14. Consequently the fieldwinding (rotor) generates magnetic flux of increasing magnitude, andthis changing flux in turn interacts with the three phases A, B, and Cof the armature windings (stator) to induce therein line-to-neutralvoltages that can be sensed and integrated by the voltage processor 48.The initial angular position of the rotor can be deduced from knowledgeof the relative polarities of the time integrals of these threevoltages.

As is indicated in FIG. 7, the subroutine 118 is entered at a pointlabeled "Find Rotor Position . . . " and then proceeds to a step 120 inwhich the microcomputer 60: (1) reads data representing the high or lowconditions of the three phase-to-phase stator voltage zero-crossingdetecting signals XA, XB, and XC on the output lines 51, 52, and 53 ofthe voltage processor 48, (2) uses a logical combination of this data tofind, in an appropriately encoded look-up table, whichever one of thesix different states 1 through 6 (see FIG. 2) is extant, and (3) placesthe number of the extant or actual state in a selected register of itsmemory where this number is saved as "oldstate." From step 120 thesubroutine proceeds to a decision point 121 which determines whether ornot the oldstate is even (i.e., 2, 4, or 6). If the oldstate is even,the next step 122 in the program inquires as to the polarity of thevoltage on the precharged commutation capacitor (as indicated by thehigh or low state of the discrete output signal VE1 from the firstvoltage sensor in the voltage processor 48), and if the polarity ispositive the control proceeds to a step 123. On the other hand, if it isdetermined that the oldstate is not even (i.e., is 1, 3, or 5), thesubroutine alternatively proceeds from step 121 to another inquiry point124 which is a duplicate of the inquiry 122, and if the polarity is notpositive the control proceeds to the same step 123.

In step 123 the microcomputer reads the oldstate, increments it by 1 tofind the number of the succeeding state, and identifies the proper one(TX) of the six main valves that is scheduled to be turned on inresponse to the next state change. For example, if oldstate=2, thesucceeding state is 3 and TX is T3. Step 123 is immediately followed bya step 125 in which the controller 44 is instructed to generate a firingsignal for the identified valve TX. Once TX is turned on by this firingsignal, it completes a path for battery current in parallel with a firstone of the two auxiliary valves that were turned on by step 114 of theSet-up Routine 83. The parallel path includes one phase of the statorwindings and the precharged commutation capacitor, and the capacitorvoltage will have the proper polarity to provide commutating action thatquickly forces the first auxiliary valve to turn off. The currentflowing in this path and through the other auxiliary valve rises to apeak magnitude and then decays to zero (in the process of which thecommutation capacitor will be discharged and then recharged with reversepolarity), and both TX and the second auxiliary valve will automaticallyturn off by self commutation when the current oscillates to zero at theconclusion of this ringing action. Now the capacitor voltage has thecorrect polarity for successful commutation of the main valves at thenext zero crossing of the phase-to-phase stator voltages during theengine cranking mode of operation. FIG. 2 illustrates that the correctpolarity is negative during even numbered states and positive during oddnumbered states.

After step 125 causes the controller to produce a firing signal forturning on TX, and while the above-described ringing action is takingplace, the field winding 18 of the machine 14 continues to be excited bycurrent which now "freewheels" through the contactor K1n and the branchof the load current path joining conductors 26 and 27 (FIG. 1). At thesame time, the subroutine shown in FIG. 7 proceeds to inquire, at point126, as to whether or not the capacitor voltage has changed polarity (asindicated by a state change of VE1). As soon as the answer isaffirmative, the control returns to the Main Routine, and the NormalCranking Routine 84 can begin.

As is shown in FIG. 7, there is an alternative way to get to the lastinquiry point 126 of the subroutine 118. It includes two steps 127 and128 that are similar to steps 123 and 125, respectively, except thatstep 127 increments the oldstate by 2 and identifies the main valve thatis scheduled to be turned on in response to the second state change tocome. For example, if oldstate=2, oldstate+2=4, and TX is T4. Step 127is initiated if the answer to inquiry 122 indicates that the capacitorvoltage is not positive, or if the answer to inquiry 124 indicates thatthe capacitor voltage is positive. It is immediately followed by thestep 128 which causes the controller to generate a firing signal for themain valve TX. When turned on, TX connects the commutation capacitoracross a first one of the conducting auxiliary valves Tp and Tn,whereupon the first auxiliary valve is forced to turn off, thecommutation capacitor is discharged then recharged with reversepolarity, and both TX and the other auxiliary valve will turn off assoon as the battery current oscillates to zero. At this time thepolarity of the capacitor voltage will not be correct for successfulcommutation of the main valves when cranking starts unless prior theretoa series of additional steps 130 through 136 are followed to reversethis polarity from wrong to right.

Step 130, which immediately follows step 128, performs the same inquiryas point 126, and as soon as the capacitor voltage changes polarity thecontrol proceeds to step 131. In step 131 the microcomputer reads theoldstate, decrements it by 1 to find the number of the preceding state,and identifies the proper pair (TX, TY) of the six main valves that arenormally conducting load current during such preceding state. Forexample, if oldstate =2, the preceding state is 1 and the valve pair TX,TY are T6 and T1, respectively. Step 131 is immediately followed by thestep 132 in which the controller is instructed to generate concurrentfiring signals for the valves TX and TY. When the valves TX and TY areturned on by these firing signals, they enable battery current to resumeflowing in the armature windings of the machine 14. After the firingsignals are generated (step 132), the control waits for a period ofapproximately 30 milliseconds to allow excitation current in the fieldwinding 18 (step 133) to increase in magnitude, and it then proceeds toa decision point 134 which determines whether or not the oldstate is aneven number. If the oldstate is even, the next step 135 is to generate afiring signal for turning on the auxiliary valve Tn, if the oldstate isnot even, an alternative step 136 is implemented to generate a firingsignal for turning on the auxiliary valve Tp, and in either case thecontrol proceeds from step 135 or 136 to the previously describedinquiry point 126. Once valve Tn (or Tp) is turned on by step 135 (or136), it connects the precharged commutation capacitor across the mainvalve TX which is thereby forced commutated off. The current thattransfers from TX to the auxiliary valve first discharges and thenrecharges the commutation capacitor with opposite polarity, and both TYand the auxiliary valve will automatically turn off by self commutationas this current oscillates to zero. Now the capacitor voltage has theright polarity for successful commutation of the main valves whencranking starts. As was previously explained, the control transfers tothe Normal Cranking Routine 84 as soon as the inquiry point 126 detectsa change in the polarity of the voltage on the commutation capacitor.

The Normal Cranking Routine 84 will cause the controller 44 to producethe proper pattern of properly synchronized firing signals as requiredfor the inverter to operate in a third harmonic commutation mode,whereby mechanical torque is developed in the rotor of the machine 14 tostart turning the crankshaft of the engine 16 and to accelerate it fromrest to a predetermined speed (e.g., 240 rpm) well above the minimum"firing speed" of the engine. The presently preferred embodiment of thisroutine is shown in FIG. 8. In its first step 140, the oldstate that wassaved in the memory of the microcomputer 60 is used to find, in anappropriately encoded look-up table, the identity of the pair of mainvalves (TX, TY) that normally should be conducting load current duringsuch state. For example, if oldstate =2, TX is T1, and TY is T2. Step140 is followed immediately by a step 141 in which the controller isinstructed to generate the proper pair of firing signals (e.g., 71 and72) to turn on both of the valves TX and TY. In practice, each firingsignal can actually comprise a 35-microsecond burst of from five to tenshort-duration (1.5 microseconds) discrete d-c signals having sufficientmagnitude to turn on the associated valve.

Once the main valves TX and TY are turned on by step 141, they completea path for load current to flow from the battery 17 through two phasesof the armature (stator) windings of the machine 14, and through thesection of the path that interconnects conductors 25 and 26 (FIG. 1).The latter section comprises the field winding 18 (which was insertedtherein when the two poles K3a and K3b of contactor K3 were closed bystep 85 of the Set-up Routine 83) and the parallel branch that includesconductor 27, resistor 28, resistor 29 shunted by contactor K2, andinductor 30. Since K2 opened at step 111 of the Set-up Routine and hasnot yet reclosed, both resistors 28 and 29 are now effectively in seriesin this parallel branch. The magnitude of armature current is initiallyvery high, limited only by the internal resistance of the battery, thenegligible resistance of the armature windings, and the total resistanceof the two resistors 28 and 29. At the time TX and TY start conductingload current, the field winding 18 is being excited by the residual ofthe current that had previously built up therein during the interval oftime between the execution of step 114 (FIG. 5) and the execution ofstep 125, 135 or 136 (FIG. 7) in the Set-up Routine. The magnetic fieldsgenerated by current in the armature windings now interact with theexcitation current in the field winding to produce in the rotor 15 atorque (proportional to the product of the magnitudes of these currents)that tends to turn the crankshaft of the engine 16 in the desireddirection.

In one practical embodiment, currents in the armature and field windingswere high enough, with the battery not fully charged, to produce a"breakaway" torque of at least 3,600 foot-pounds which is sufficient toturn the crankshaft of a 4,000 horsepower diesel engine. As thecrankshaft and rotor start rotating, current (and torque) tends todecrease in magnitude due to the rising amplitude of the back emf thatis induced in the armature windings and that opposes the batteryvoltage. The instantaneous magnitude of the back emf in each of thethree phases of the synchronous machine will alternate sinusoidallybetween relatively positive and negative peaks as the rotor acceleratesfrom rest and its angular position advances. In due course the rotorwill pass through a location where the increasing voltage magnitude ofthe oncoming or relieving phase (e.g., B) just equals the decreasingvoltage magnitude of the offgoing or relieved phase (e.g., A), whereuponone of the three zero-crossing detecting signals (e.g., XC) will changeup or down to mark the transition to the next state.

As is shown in FIG. 8, after generating the firing signals for valves TXand TY (step 141), the Normal Cranking Routine proceeds to a step 142 inwhich a K2 timer is started. This timer will run for a predeterminedlength of time (e.g., approximately three seconds) after being started.Step 142 is followed by a step 143 in which the microcomputer: (1) readsdata representing the high or low conditions of the signals XA, XB, andXC, (2) uses a logical combination of this data to find, in anappropriately encoded look-up table, whichever one of the six differentstates 1 through 6 is extant, and (3) places the number of the extant oractual state in its memory where this number is saved as "new state."From step 143 the control proceeds to an inquiry point 144 whichdetermines whether or not the new state is the same as oldstate. So longas the answer is affirmative, the control next inquires, at a point 145,as to whether or not a cranking "finish" flag is on, and if not it thenproceeds to an inquiry point 146 where the open or closed status ofcontactor K2 is tested. If K2 is closed, the control returns to step143; if not, the status of the K2 timer is tested at point 147. Assumingthat the K2 timer is still running, the control immediately returns tostep 143. Otherwise, the control proceeds from the inquiry point 147 toa step 148 in which the contactor K2 is commanded to close, whereuponthe control returns to step 143. It will now be apparent that so long asthe inquiry step 144 determines that the new state is the same asoldstate, the control steps repetitively around a loop comprising thestep 143 and inquiry points 144, 145, 146, and 147 while the K2 timer isrunning, whereas it steps repetitively around a subloop comprising step143 and inquiry points 144, 145, and 146 after the length of timeprogrammed in the K2 timer is over. In response to the expiration ofthis predetermined length of time, step 148 is implemented to close thecontactor K2 which then short circuits the resistor 29 (FIG. 1), therebyreducing the ohmic value of the resistance in parallel with the fieldwinding 18. As a result, more current can flow in the branch of the loadcurrent path between conductors 26 and 27, the field excitation isweakened, and higher cranking speeds can be achieved.

As soon as the inquiry point 144 determines that the new state is notthe same as the oldstate (i.e., in response to a state change of one ofthe phase-to-phase stator voltage zero-crossing detecting signals XA,XB, and XC), the control transfers from the above-described loop to analternative loop comprising the step 143, the inquiry point 144, and, inthe following order, a "Commutation Subroutine" 151, a "Next StateSubroutine" 152, an inquiry point 153, and a "System Synch Subroutine"154. The three subroutines 151, 152, and 154 are therefore executed eachtime the inquiry point 144 detects a state change.

The Commutation Subroutine 151 of the Normal Cranking Routine 84 isshown in FIG. 9. It has two functions: (1) to initiate commutation ofthe offgoing main valve by ordering the production of a firing signalfor the appropriate one of the two auxiliary valves Tp and Tn, and (2)to delay the execution of the Next State Subroutine 152 until aprogrammed interval of time has elapsed following each resulting zerocrossing of the voltage on the commutation capacitor 45. The CommutationSubroutine is entered at a point labeled "Commutation" and then proceedsto a decision point 156 which determines whether or not oldstate was anodd number. If oldstate was odd (i.e., 1, 3 or 5), the control proceedsto a step 157 in which the controller is instructed to produce a firingsignal 69 for turning on the auxiliary valve Tn. If oldstate was even(2, 4 or 6), the control proceeds from point 156 to a step 158 in whichthe controller is instructed to produce a firing signal 68 for turningon the auxiliary valve Tp. Consequently one or the other of theauxiliary valves is turned on to connect the commutation capacitoracross the offgoing main valve. The capacitor voltage will now have thecorrect polarity to force load current to transfer to the conductingauxiliary valve, whereupon the offgoing main valve stops conducting. Theload current in the commutation circuit first discharges the capacitorand then recharges it with reverse polarity, as is illustrated in thepreviously described FIG. 3. More specifically, if oldstate was odd, Tnis fired, and at time t3 the capacitor voltage will change from positiveto negative as indicated by a high-to-low state change of the outputsignal VE1 of the first bistable capacitor voltage sensor in the voltageprocessor 48. On the other hand, if oldstate was even, Tp is fired, andat time t8 the capacitor voltage will change from negative to positiveas indicated by a low-to-high state change of VE1.

As is shown in FIG. 9, step 157 (or 158) of the Commutation Subroutine151 is immediately followed by an inquiry, at point 160 (or 161), as tothe high or low state of the capacitor voltage polarity indicatingsignal VE1. As soon as VE1 changes state, the control proceeds frompoint 160 (or 161) to another inquiry point 162 where the status of theK2 timer is measured. For a predetermined initial period of time, whichstarts when step 142 of the Normal Cranking Routine 84 starts the K2timer, the inquiry 162 yields a negative answer, and thereafter theanswer will be affirmative. The initial period is preferablyapproximately 1.5 seconds, or approximately half of the length of timethat the K2 timer is programmed to run.

In response to a negative answer at the inquiry point 162, the next step163 in the program is to load a predetermined maximum capacitorrecharging time into a fourth timer. Step 163 is followed immediately bya step 164 that starts the fourth timer and then by an inquiry, at apoint 165, as to the state of the second capacitor voltage sensor in thevoltage processor 48, as indicated by the high or low state of thesignal VE2. So long as the capacitor voltage is lower than thepredetermined maximum level (e.g., 400 volts), the answer to inquiry 165is negative, and the control proceeds to an inquiry point 166 where thestatus of the fourth timer is tested. So long as the timer #4 is stillrunning, the answer to inquiry 166 is negative, and the control returnsto the preceding inquiry point 165. But whenever an affirmative answeris obtained at either inquiry point 165 (revealing that the magnitude ofcapacitor voltage has attained the aforesaid maximum) or inquiry point166 (revealing that the time delay interval of the timer #4 is over),whichever is first to occur, the Commutation Subroutine 151 is exited,and the control is transferred to the Next State Subroutine 152 of theNormal Cranking Routine. Preferably the maximum recharge time that isloaded into timer #4 in step 163 is selected to have a relatively long,fixed duration (e.g., approximately 1.2 milliseconds) so that anaffirmative answer will ordinarily be obtained from inquiry 165 earlierthan from inquiry 166 throughout the aforesaid initial period (i.e.,before the inquiry point 162 yields an affirmative answer). In otherwords, whenever the Commutation Subroutine is executed during theinitial period of time, it will be completed as soon as the voltage onthe commutation capacitor reverses polarity and rises in magnitude tothe aforesaid maximum level (but no later than the expiration of themaximum recharging interval that was loaded in timer #4 at step 163).High capacitor voltage is required for successful commutation duringthis period when load current is relatively high. At the same time, therelatively long maximum capacitor recharging time is permissible becausethe rotor speed (and hence the frequency of state changes) is nowrelatively low.

In response to an affirmative answer at the inquiry point 162 (whichwill be true anytime the Commutation Subroutine 151 is executed afterexpiration of the aforesaid initial period of time), the program shownin FIG. 9 proceeds from point 162 to a decision point 167 which is aduplicate of 156. If oldstate was odd, the next step 168 is to load a"negative" capacitor recharging time into timer #4. Alternatively, ifoldstate was even, the next step 169 is to load a "positive" capacitorrecharging time into the same timer. In either case, the control thenproceeds, as before, to start timer #4 at step 164 and then repetitivelyto check for maximum capacitor voltage at point 165 and to test thestatus of the timer at point 166. In accordance with the presentinvention, the delay intervals that are loaded into timer #4 at steps168 and 169 are shorter than the maximum capacitor recharging intervalthat is loaded at step 163. While both intervals could be equal to eachother if desired, in the illustrated embodiment the negative recharginginterval is shorter than the positive recharging interval. By way ofexample, the negative recharging interval is approximately 300microseconds, and the positive recharging interval is approximately 500microseconds. These intervals, which in FIG. 3 are respectivelyrepresented by the delta t- and t+ symbols, are sufficiently short sothat, after the aforesaid initial period expires (i.e., when the inquirypoint 162 yields an affirmative answer), an affirmative answer willordinarily be obtained from inquiry 166 earlier than from inquiry 165.In other words, whenever the Commutation Subroutine is executed afterthe initial period of time, it is completed as soon as the delayinterval that was loaded in timer #4 at step 168 or 169 is over, andthis does not provide enough time for the commutation capacitor torecharge to the aforesaid maximum level of voltage. Consequently, as wasexplained hereinbefore in connection with the description of FIG. 3, theactual magnitude of capacitor voltage at the conclusion of theCommutation Subroutine is a function of the magnitude of load current.It decreases with current, and as a result the length of the commutationinterval is desirably reduced as the rotor speed (and frequency)increases after the aforesaid initial period of time.

The Next State Subroutine 152, which is executed immediately after theCommutation Subroutine 151, is shown in FIG. 10. Its purposes are (1) tocalculate and save the "next state" and (2) to complete the thirdharmonic commutation process by ordering the production of a firingsignal for the oncoming or relieving main valve. This subroutine isentered at a point labeled "Next State" and then proceeds to a step 171in which the microcomputer (1) subtracts the previously saved oldstate(step 120 in FIG. 7) from the previously saved new state (step 143 inFIG. 8) to find the difference therebetween, the difference being +1 forthe assumed phase rotation A-B-C of stator voltages but -1 if the phaserotation were C-B-A, (2) increments the oldstate by one if thedifference is +1 (or decrements it if the difference were -1) to givethe number of the "next state," i.e., the state which comes afteroldstate and which therefore should coincide with the new state, and (3)places the number of the "next state" in the selected register of itsmemory where this number replaces the previously saved oldstate and issaved as a calculated new oldstate. Step 171 is followed by a step 172in which the new oldstate is used to find, in an appropriately encodedlook-up table, the identity of the pair of main valves (TX, TY) thatnormally should be conducting load current during such state. Forexample, if new oldstate =3, TX is T2, and TY is T3. TY is the oncomingor relieving valve of the pair. Step 172 is followed immediately by astep 173 in which the controller is instructed to generate the properpair of firing signals (e.g., 72 and 73) to turn on the identifiedvalves TX and TY. It will be apparent that the firing signal for valveTX is redundant, as TX in this subroutine is the same valve as TY whichwas turned on earlier in the program. Once the oncoming valve TY isturned on by a firing signal produced at step 173, load current cantransfer to it from the auxiliary valve that was turned on by step 157or 158 of the Commutation Subroutine 151 (FIG. 9), thereby completingthe commutation process. While load current is decaying to zero in theauxiliary valve, the commutation capacitor continues recharging to apeak magnitude somewhat beyond its level of voltage at the conclusion ofthe Commutation Subroutine.

The firing signal that is produced by step 173 of the Next StateSubroutine 152 for the oncoming main valve TY is represented in FIG. 3by the pointer 7X. It is produced at time t4 if the calculated newoldstate is an even number and at time t9 if odd. Both of these timesare delayed with respect to the preceding state change of the capacitorvoltage polarity indicating signal VE1 (as detected by the inquiry point160 or 161 of the Commutation Subroutine 151). Each time the Next StateSubroutine is executed during the aforesaid initial period (which isdetermined by step 162 of the Commutation Subroutine), this delay willdepend on how long the commutation capacitor takes to recharge to thevoltage level at which an affirmative answer is obtained at the inquirypoint 165 in the Commutation Subroutine. But each time the Next StateSubroutine is executed after the initial period, the firing signal 7X isdelayed until a programmed interval of time has elapsed. If the newoldstate is an even number, the programmed delay interval is determinedby the negative recharging time loaded into timer #4 at step 168 of theCommutation Subroutine, and otherwise it is determined by the positiverecharging time loaded into the same timer at step 169.

Having generated the firing signal (7X) for the main valve TY at step173, the Next State Subroutine 152 returns to the Normal CrankingRoutine (FIG. 8) where the calculated new oldstate (step 171 in FIG. 10)is checked, at the inquiry point 153, to be sure that it is in fact thesame as the previously saved new state (step 143 in FIG. 8). If not,after waiting for a fixed period of approximately 50 milliseconds (step174), the control is retransferred to the Commutation Subroutine 151,and the two subroutines 151 and 152 are executed again. This process isrepeated, if necessary, until the calculated new oldstate coincides withthe new state, and then the control is transferred to the System SynchSubroutine 153.

The presently preferred embodiment of the System Synch Subroutine isshown in FIG. 11. It is entered at a point labeled "System Synch" andproceeds to an inquiry point 175 where the open or closed status of thecontactor K2 is checked. Initially, while the K2 timer is running (seesteps 142 and 147 in FIG. 8) and therefore prior to implementation ofthe step 148 that commands the contactor K2 to close, this inquiry willreveal that K2 is open. Consequently, after waiting for a certain delayinterval (step 176) the control is returned directly to the step 143 ofthe Normal Cranking Routine 84. The delay introduced by step 176 willallow time, after implementing step 173 of the Next State Subroutine(FIG. 10) and before returning to the step 143, for the above-describedcommutation from the auxiliary valve (Tp or Tn) to the oncoming mainvalve TY to be completed and for the resulting electrical transients("noise") to subside in the voltage processor 48. The delayed return tostep 143 is desirable when load current is relatively high (as is trueinitially), because the aforesaid transients might then be severe enoughto cause false data to be supplied on lines 51, 52 and 53 to thecontroller 44. Once the control returns from step 176 (FIG. 11) to step143 (FIG. 8), the loop comprising step 143 and inquiry points 144, 145,146, and 147 is repeatedly executed until the next state change isindicated by a negative answer to inquiry 144 (i.e., until the newstate, as determined by step 143, is no longer the same as the newoldstate that was calculated by step 171 of the Next State Subroutine152), whereupon the control again transfers to the CommutationSubroutine 151.

As the angular position of the rotor of the machine 14 advances withincreasing speed during the cranking mode of operation, theabove-described execution of the steps 143-147 in the Normal CrankingRoutine 84 and of the subroutines 151-154 are automatically repeateduntil the K2 timer stops running. The Commutation Subroutine 151 isinitiated each time the inquiry 144 indicates a state change, and suchchanges occur with increasing frequency as the rotor accelerates. Inpractice it make take approximately two seconds for the rotor tocomplete its first revolution and another second for a secondrevolution. By the end of two revolutions the rotor may have attained aspeed on the order of 100 rpm, and it is around this time that the K2timer stops running and the contactor K2 is closed to further weaken thefield and permit higher speed cranking.

Each time the System Synch Subroutine 154 is executed after contactor K2is closed, the control will proceed from inquiry point 175 to an inquirypoint 177 in which the status of the first timer is tested. As shown inFIG. 11, if timer #1 has stopped running the next step 178 of thissubroutine will cause the controller 44 to command all contactors toopen, and the Normal Cranking Routine is then aborted at a stop point179. Assuming, however, that timer #1 is still running, the controlproceeds from point 177 to a speed check step 181 where the rotor speedis measured. Any suitable means can be used for this purpose. One simpleyet effective means for measuring rotor speed is to count the number oftimes the Commutation Subroutine 151 is executed over a known period oftime. It can be shown that this count is proportional to speed. Thereare six state changes and hence six commutation intervals per cycle ofthe fundamental component of alternating voltages on the stator windingsof the machine 14, and one complete revolution of the rotor correspondsto five such cycles in a 10-pole machine. Thus the predeterminedthreshold speed of 240 rpm corresponds to a fundamental frequency of 20Hertz which is indicated if 12 commutations are counted in a period of0.1 second.

From step 181 the System Synch Subroutine proceeds to a point 182 thatinquires as to whether or not the rotor speed has attained apredetermined rate (i.e., the aforesaid threshold speed of 240 rpm). Ifnot, the control is then returned directly to the step 143 of the NormalCranking Routine 84 (FIG. 8). Now the subloop comprising step 143 andinquiry points 144, 145, and 146 will be repeatedly executed until thenext state change takes place, whereupon the control once againtransfers to the Commutation Subroutine 151.

The above-described execution of the steps 143-146 and of thesubroutines 151-154 are automatically repeated until the rotor isrotating faster than the aforesaid predetermined rate. Once thisthreshold speed is exceeded, the inquiring point 182 of the System SynchSubroutine (FIG. 11) yields an affirmative answer. In this event thecontrol proceeds from point 182 to a step 183 that causes the controllerto issue an appropriate signal that the engine is running. Step 183 isfollowed immediately by a step 184 that sets the cranking finish flag inan "on" state, and later by a step 185 that will cause the controller tocommand the opening of contactors K1p, K1n K2 and K3. From step 185 thecontrol returns to step 143 of the Normal Cranking Routine (FIG. 8). Nowthe step 143 and the inquiry points 144 and 145 are passed throughagain, and from the inquiry point 145 the control can proceed to afinish point 186 which marks the conclusion of the cranking mode ofoperation.

While a preferred embodiment of the invention has been shown anddescribed by way of example, many modifications will undoubtedly occurto persons skilled in the art. The concluding claims are thereforeintended to cover all such modifications as fall within the true spiritand scope of the invention.

We claim:
 1. An improved third harmonic auxiliary impulse commutatedelectric power inverter that is operative to supply variable frequencyalternating current to the three different phases of a 3-phase inductiveload circuit from a source of relatively smooth direct current, saidinverter including at least three pairs of alternately conducting maincontrollable electric valves arranged in a 3-phase, double-way bridgeconfiguration for interconnecting said source and said load circuit, anda commutation circuit including at least first and second auxiliarycontrollable electric valves interconnected in series aiding fashionacross said source and connected via a precharged commutation capacitorto said load circuit, wherein the improvement comprises:a. bistablefirst means coupled to said capacitor for sensing the electricalpotential difference across said capacitor, said first means being inone state whenever the potential on one side of the capacitor ismeasurably positive with respect to the other side and being in adifferent state whenever said potential is measurably negative; b.second means coupled to said 3-phase load circuit for detecting all zerocrossings of the fundamental phase-to-phase alternating voltages thatare developed at line terminals of the respective phases of said loadcircuit; c. control means coupled to both said first and second meansand having a third harmonic commutation mode of operation in which itcyclically produces a family of periodic firing signals that cause saidvalves to turn on selectively, said family including a first series offiring signals respectively produced in response to said second meansdetecting consecutive zero crossings of said phase-to-phase voltages foralternately turning on said auxiliary valves, whereupon load current canimmediately transfer from an offgoing main valve to a parallel pathincluding the turned-on auxiliary valve and said capacitor which isfirst discharged and then recharged with reverse polarity by suchcurrent, and said family also including a second series of firingsignals respectively produced in delayed response to successive statechanges of said first means for turning on said main valves in apredetermined sequence, whereupon load current can then transfer to theoncoming main valve from the turned-on auxiliary valve; and d. saidcontrol means including time delay means effective at least after apredetermined initial period of time, which starts when said thirdharmonic commutation mode of operation commences, for delaying theproduction of each firing signal in said second series until aprogrammed interval of time has elapsed following each state change ofsaid first means.
 2. The inverter as in claim 1, in which said loadcircuit comprises the stator of windings of a rotatable synchronousmachine.
 3. The inverter as in claim 2, in which said stator windingsare star connected and said capacitor is connected between the neutralof said windings and the juncture of said auxiliary valves.
 4. Theinverter as in claim 2, in which said source comprises an electricstorage battery.
 5. The inverter as in claim 2, in which load currentflows between said source and said inverter through an impedance thathas appreciable electrical inductance.
 6. The inverter as in claim 5, inwhich said impedance comprises the field winding of said machine.
 7. Theinverter as in claim 6, in which said source comprises an electricstorage battery.
 8. An inverter as in claim 1, in which voltage sensingmeans is coupled to said capacitor for detecting whether or not thecapacitor voltage has a magnitude exceeding a predetermined level, andin which said control means is so arranged that each firing signal insaid second series is produced, after said first mean changes state, inresponse to either (i) the elapse of the programmed delay interval or(ii) the voltage sensing means detecting that said capacitor has beenrecharged to a voltage magnitude in excess of said predetermined level,whichever is first to occur.
 9. An inverter as in claim 8, in which saidtime delay means is programmed so that throughout said predeterminedinitial period of time it will delay each firing signal in said secondseries for an interval of preselected fixed duration, and thereafter thedelay intervals are shorter than said preselected fixed duration.
 10. Aninverter as in claim 9, in which said initial period of time isapproximately 1.5 seconds, said preselected fixed duration isapproximately 1.2 milliseconds, and said shorter delay intervals do notexceed approximately 0.5 millisecond.
 11. An inverter as in claim 9, inwhich said time delay means is additionally programmed so that aftersaid initial period of time the duration of alternate delay intervalsdiffers from the duration of intermediate delay intervals.
 12. Aninverter as in claim 11, in which said alternate delay intervals have ashorter duration than said intermediate delay intervals.
 13. An inverteras in claim 1, in which said second means has six different states forindicating, during each cycle of operation, the six differentcombinations of relative polarities of said phase-to-phase voltages,said second means experiencing a state change each time the magnitude ofany of said phase-to-phase voltages crosses zero, and in which saidcontrol means is effective to produce firing signals for turning on thesecond auxiliary valve in immediate response to said second meanschanging from a first state to a second state and from a third state toa fourth state and from a fifth state to a sixth state, said controlmeans being similarly effective to produce firing signals for turning onthe first auxiliary valve in immediate response to said second meanschanging from said sixth to said first state and from said second tosaid third state and from said fourth to said fifth state.
 14. Aninverter as in claim 13, in which said time delay means is programmed sothat throughout said predetermined initial period of time it iseffective to delay each firing signal in said second series for aninterval of preselected fixed duration, and thereafter the delayintervals are shorter than said preselected fixed duration.
 15. Aninverter as in claim 14, in which said time delay means is additionallyprogrammed so that after said initial period of time the duration ofalternate delay intervals is shorter than the duration of intermediatedelay intervals.